1. Technical Field
The invention relates to evaluating the safety margin offered by a digital video signal, on a parallel digital video interface, in case of phase variation between a clock signal and the data signals circulating in parallel relation. It finds a particularly important application in the construction of test tools for evaluating digital television equipment in accordance with recommendation 601 (studio digital television coding parameters) and 656 (interface for components of digital video signals for 525 and 625 line television systems) of the C.C.I.R.
Recommendation 656 of the C.C.I.R. defines a parallel digital video interface for passing of a digital video signal comprising a clock signal, formed by a 27 MHz square wave, and data signals which represent a data word bit at each up-going transition of the clock signal.
Evaluation of the safety margin of such a signal or of similar signals with respect to different types of degradation is useful in order to determine whether a particular piece of equipment, to be qualified, may be used in an equipment chain forming an installation. In fact, receiving equipment may be acceptable because it correctly decodes the data transmitted by a piece of equipment to be qualified, when it is located immediately at the output thereof. But such receiving equipment will however not operate correctly if it is separated from the equipment to be qualified by transmission equipment introducing additional degradations, for example a phase shift between the clock signal and the data signals or attenuation.
2. Prior Art
Different methods have already been proposed for evaluating the additional phase shift between the clock and data which the digital video signal received by an interface may accept without detrimentally affecting reception, i.e. the safety margin offered by the digital video signal.
A prior art method consists in measuring the maximum phase shift, with respect to reference positions, existing between each of the data signals and the clock signal, by using a two channel oscilloscope. But the evaluation thus obtained lacks accuracy, for the largest phase shift between the clock and data may occur very unfrequently, for example due to randomly occuring phenomena (phase jitter) and intersymbol interferences, and be hardly noticeable.
Another solution consists in displaying, for each of the data signal carriers, the eye pattern and its position with respect to the up-going edges of the clock signal and in deriving therefrom an evaluation of the tolerable additional phase shift. But obtaining the eye pattern generally involves applying a pseudo-random bit sequence to the input so as to explore all intersymbol interference (ISI) phenomena over all possible bit sequences. Although delivering a pseudo-random bit sequence to the input of the equipment is possible for certain equipments, such as a cable or a switching grid, it is not always so for an equipment which is a source of images, such a camera having an encoder.
Finally, these two methods involve making as many tests as there are data signal carriers in the interface.